Glass Transition Temperature Analysis of PEEK Films: Testing Methods, Standards, and Industrial Appl

PEEK (polyetheretherketone) film is a high-performance semi-crystalline thermoplastic whose glass transition temperature (Tg, approximately 143°C) fundamentally governs its thermal stability, mechanical modulus, and long-term service reliability. This article provides a comprehensive technical analysis of PEEK film Tg measurement, covering testing samples, testing items, measurement methods such as DSC and DMA, influencing factors, and typical industrial applications.

Technical Overview

PEEK film is a semi-crystalline, aromatic thermoplastic composed of repeating units containing ketone and ether functional groups linked to phenylene rings. This molecular architecture delivers an exceptional combination of thermal resistance, chemical inertness, mechanical strength, and electrical insulation performance. Among its thermal characteristics, the glass transition temperature is one of the most critical parameters for material qualification and process design.

The glass transition occurs within the amorphous regions of the polymer, where chain segments transition from a frozen, glassy state to a mobile, rubbery state as temperature increases. For PEEK, the Tg is typically reported between 140°C and 150°C, with a midpoint value of approximately 143°C when measured by differential scanning calorimetry (DSC). This transition produces measurable changes in:

• Heat capacity (step change observable by DSC)
• Storage modulus (drop of one to two orders of magnitude observable by DMA)
• Coefficient of thermal expansion (sharp increase above Tg)
• Dielectric properties and ionic mobility
• Creep resistance and dimensional stability

Because PEEK is semi-crystalline, its measured Tg depends strongly on crystallinity, thermal history, molecular weight, and processing conditions. Accurate Tg analysis therefore requires carefully controlled sample preparation, standardized measurement protocols, and correct interpretation of transition region data. Since the amorphous fraction is constrained by surrounding crystalline lamellae, films with higher crystallinity often exhibit a broader and slightly shifted glass transition.

Testing Samples

Unfilled Neat PEEK Films

Standard-grade PEEK films with film thicknesses typically ranging from 6 micrometers to 500 micrometers represent the most common test specimens. For DSC analysis, specimens of 5 to 10 milligrams are punched or cut from the film and hermetically sealed in aluminum pans to ensure uniform thermal contact and to suppress oxidative degradation during heating.

Modified and Reinforced PEEK Films

Filler-modified PEEK films, including glass-fiber filled, carbon-filled, PTFE-lubricated, and mineral-filled grades, require additional attention during sampling. The specimen must be representative of the filler loading, and the measured heat capacity step at Tg must be normalized to the actual polymer fraction rather than total sample mass to avoid artificially suppressing the transition signal.

Special Grades and Blends

High-temperature grades, such as PEK or PEKK copolymers, and PEEK blends may exhibit Tg values ranging from 150°C to 175°C. These samples should be tested with an extended temperature program to capture the complete transition region. Thermally aged or annealed film samples should also be conditioned and documented, since thermal history directly affects the observed Tg.

Testing Items

A complete glass transition temperature analysis of PEEK film typically includes the following test items:

• Glass transition temperature reported as extrapolated onset, midpoint (half-height), and endset temperatures per ASTM E1356 and ISO 11357-2
• Specific heat capacity step (delta Cp) at the glass transition
• Melting temperature (Tm) and melting enthalpy for crystallinity correlation
• Cold crystallization temperature (Tcc), observed in quenched amorphous films
• Degree of crystallinity calculated from melting enthalpy normalized to 100% crystalline PEEK reference value (approximately 130 J/g)
• Storage modulus, loss modulus, and tan delta peak temperatures by DMA
• Dimensional change rate and coefficient of thermal expansion by TMA

Reporting both first-heating and second-heating scans is standard practice. The first heating reflects the as-received thermal history of the film, while the second heating, after controlled cooling, provides an intrinsic material property suitable for lot-to-lot comparison.

Testing Methods

Differential Scanning Calorimetry (DSC)

DSC is the primary and most widely accepted method for Tg determination of PEEK films. Testing is performed under nitrogen purge (40 to 60 mL/min) with sample masses of 5 to 10 mg at heating rates of 10°C/min or 20°C/min, following ASTM D3418, ASTM E1356, and ISO 11357-2. The glass transition is identified as the stepwise increase in heat capacity, and the midpoint method is the most commonly reported value. For thin films with low amorphous content, modulated temperature DSC (MTDSC) can improve resolution by separating reversing and non-reversing heat flow components.

Dynamic Mechanical Analysis (DMA)

DMA measures the viscoelastic response of PEEK film under oscillatory loading and is considerably more sensitive to the glass transition than DSC. Thin-film tension fixtures are used with specimen strips typically 5 to 6 mm wide, tested at frequencies of 1 Hz with small strain amplitudes within the linear viscoelastic region, following ASTM D4065 and ISO 6721. The tan delta peak, typically appearing between 155°C and 175°C, defines the mechanical glass transition. Because DMA probes molecular relaxation at a finite frequency, DMA-derived Tg values are systematically 10 to 25°C higher than DSC midpoint values.

Thermomechanical Analysis (TMA)

TMA detects the glass transition through the change in slope of the dimension-versus-temperature curve caused by the abrupt increase in thermal expansion coefficient above Tg. A quartz expansion probe with a small static load is applied to a stacked film specimen under nitrogen. TMA is particularly useful for evaluating the in-plane and through-plane dimensional behavior of PEEK films used in flexible electronics, where the CTE step at Tg directly affects circuit reliability.

Applications

Glass transition temperature analysis of PEEK film supports qualification and process control across demanding industries:

• Flexible printed circuits and high-temperature adhesiveless copper clad laminates, where the modulus retention above Tg determines bending performance
• Aerospace insulation films and composite interleaf materials requiring certified thermal performance data
• Medical device components and sterilization-resistant films validated against repeated autoclave exposure near the Tg
• High-temperature capacitor and battery insulation films where dielectric stability across the transition is critical
• Wire and cable wrapping tapes and semiconductor carrier films exposed to elevated processing temperatures
• Research and development of modified PEEK grades, copolymers, and multilayer film structures

In each application, Tg data establishes safe processing windows, predicts long-term service behavior, and supports failure analysis when films experience unexpected thermal excursions.

FAQ

What is the typical glass transition temperature of PEEK film?

The Tg of standard PEEK film is approximately 143°C by DSC midpoint analysis, with typical ranges between 140°C and 150°C depending on crystallinity and thermal history. Special high-temperature grades can reach 160°C to 175°C.

Why does DMA report a higher Tg than DSC for the same film?

DMA measures the mechanical relaxation response at an applied frequency, which shifts the apparent transition to higher temperatures due to time-temperature superposition. A DMA tan delta peak of 160°C to 170°C commonly corresponds to a DSC midpoint of about 143°C. Both values are valid but must not be compared interchangeably.

How does thermal history affect the measured Tg of PEEK film?

Rapid quenching produces amorphous-rich films that may show a lower, sharper Tg along with a cold crystallization exotherm, while slow cooling or annealing increases crystallinity, broadens the transition, and can shift the onset slightly upward. For this reason, comparing second-heating scans is recommended for intrinsic material evaluation.

Can PEEK film be used continuously at temperatures above its Tg?

Yes. Because PEEK is semi-crystalline, its crystalline regions maintain structural integrity well above Tg. PEEK film sustains continuous service temperatures of approximately 240°C to 260°C, far above its glass transition, although modulus and creep resistance decrease measurably once Tg is exceeded.

What sample preparation precautions are required for accurate Tg measurement?

Film specimens should be flat, free of contamination, punched to ensure uniform pan contact, sealed in hermetic aluminum pans, and tested promptly under nitrogen. Consistent sample mass, heating rate, and purge conditions are essential for reproducible results compliant with ASTM and ISO requirements.